The present invention relates generally to printing mechanisms, such as inkjet printers or inkjet plotters. Printing mechanisms often include an inkjet printhead which is capable of forming an image on many different types of media. The inkjet printhead ejects droplets of colored ink through a plurality of orifices and onto a given media as the media is advanced through a printzone. The printzone is defined by the plane created by the printhead orifices and any scanning or reciprocating movement the printhead may have back-and-forth and perpendicular to the movement of the media. Conventional methods for expelling ink from the printhead orifices, or nozzles, include piezo-electric and thermal techniques which are well-known to those skilled in the art. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, the Hewlett-Packard Company.
In order to achieve a high level of image quality in an inkjet printing mechanism, it is often desirable that the printheads have: consistent and small ink drop size, consistent ink drop trajectory from the printhead nozzle to the print media, and extremely reliable inkjet nozzles which do not clog. To this end, many inkjet printing mechanisms contain a service station for the maintenance of the inkjet printheads. These service stations may include scrapers, ink-solvent applicators, primers, and caps to help keep the nozzles from drying out during periods of inactivity. Additionally, inkjet printing mechanisms often contain service routines which are designed to fire ink out of each of the nozzles and into a waste spittoon in order to prevent nozzle clogging.
Despite these preventative measures, however, there are many factors at work within the typical inkjet printing mechanism which may clog the inkjet nozzles, and inkjet nozzle failures may occur. For example, paper dust may collect on the nozzles and eventually clog them. Ink residue from ink aerosol or partially clogged nozzles may be spread by service station printhead scrapers into open nozzles, causing them to be clogged. Accumulated precipitates from the ink inside of the printhead may also occlude the ink channels and the nozzles. Additionally, the heater elements in a thermal inkjet printhead may fail to energize, despite the lack of an associated clogged nozzle, thereby causing the nozzle to fail.
Clogged or failed printhead nozzles result in objectionable and easily noticeable print quality defects such as banding (visible bands of different hues or colors in what would otherwise be a uniformly colored area) or voids in the image. In fact, inkjet printing systems are so sensitive to clogged nozzles, that a single clogged nozzle out of hundreds of nozzles is often noticeable and objectionable in the printed output.
It is possible, however, for an inkjet printing system to compensate for a missing nozzle by removing it from the printing mask and replacing it with an unused nozzle or a used nozzle on a later, overlapping pass, provided the inkjet system has a way to tell when a particular nozzle is not functioning. In order to detect whether an inkjet printhead nozzle is firing, a printing mechanism may be equipped with a low cost ink drop detection system, such as the one described in U.S. Pat. No. 6,086,190 assigned to the present assignee, Hewlett-Packard Company. This drop detection system utilizes an electrostatic sensing element which is imparted with an electrical stimulus when struck by a series of ink drop bursts ejected from an inkjet printhead.
In practical implementation, however, this electrostatic sensing element has some limitations. The sensing element may adversely react with ink residue formed as a result of contact with the ink drop bursts. Additionally, drop detect signals provided from the sensing element to the sensing electronics may easily subjected to noise due to their small amplitudes. Furthermore, the ink residue remains conductive and can short-circuit the sensing electronics.
Therefore, it would be desirable to have an electrostatic sensing element and related electronics which have a substantial immunity to the potentially harmful effects of conductive ink residue and which may easily be integrated into various printing mechanism designs. It would also be desirable to have a method of efficiently and economically constructing such an electrostatic sensing element and electronics.